losing a cylinder?

[JPD]

hello everyone, new to iboats.
I have a 20 hp johnson model number: 20R70E
here is my problem, I think i am losing a cylinder at all rpm's intermittently, it is not consistant, it may miss on just one cycle or for long periods of time and the reverse is true as well. It may run just fine for a 1/4 mile or so before it misses again
here is what i have done
cleaned the carb, replaced plugs, checked wires, replaced tank, adjusted points,

I am at a loss, any help would be appreciated

 

[Mntom]

Re: losing a cylinder?

Could be a coil failing. Try hooking up a timing light to each cylinder and see if the spark does quit. It is still possible that a plug wire could be breaking down too.

 

[jonesg]

Re: losing a cylinder?

If ignition is good , rebuild carb with a carb kit/s.
Cleaning doesn't work, they need new parts and seals.

 

[Cofe]

Re: losing a cylinder?

If it is the bottom cylinder, your fuel pump diaphragm may be leaking fuel back into the pulse passage to the lower cylinder.

If there are signs of water on your plugs, it could be your head gasket, or exhaust chamber water passage gaskets.

 

[JPD]

Re: losing a cylinder?

I have checked for spark with a new spark tester thingamajigger that i got from auto zone and i got no spark at all on the top cylinder and only a couple of sparks from the bottom cylinder, and when i say a couple i mean two, (and yes i have changed the plugs)

So, i have ordered a manual, carb kit, and a new coil

We shall see. In the meantime any tips or suggestions would be extremely helpful, seeing as i have never dug into a boat motor before. ATV's, lawn mowers, weed eaters, chain saws, that type i have been pretty successful tinkering with. So i figure a boat motor should not be too hard.

But I could be wrong

and it seems the top cylinder has less spark than the bottom one

 

[JPD]

Re: losing a cylinder?

also, how do i replace the wires, will it be self explanitory when i take the flywheel off or do i need to know more about it

points: what the heck are they and what do they do? ( i assume they perform the same function as a distributor or magneto) Early form of the magneto? but........

Thanks a lot for the help so far

 

[jonesg]

Re: losing a cylinder?

Look at the coils , examine for cracks.

If they are original coils just replace them, 42 yrs old?, forgetaboutit.

Its a points system?
I'd replace everything, its cheap enough and simple.